rough running 912ULS
I am just after some ideas for what might be causing my recent rough running on my 912 ULS. The engine is in a homebuilt and both engine and airframe have done about 400 hours. Recently the engine has been running a bit rough and it seems to be exclusively on hot days. Originally I thought it was an issue with the carbs so I tore them down a few times and never found an issue with them. After this continued to happen a couple more times I decided to just bite the bullet and purchased 2 brand new carbs. The engine didn’t have any more issues after this and I thought the problem was solved, but the weather more recently has been a bit cooler. Last week it was hot again and the issue came up again after my takeoff climb. The rough running wasn’t that alarming and was similar to if you turned off a mag bank with the engine on idle….a bit rough and some loss of power but it didn’t seem like it was going to quit or anything. The gauges were all still fine but since it seemed to only happen on hot days I dropped the oil cooler down a bit to get some cooler air from under the cowling and also replaced the spark plugs in case the old carbs caused fouling of any of them. Today I flew it again. It was a hot day, 35C/ 95F so I did an enroute climb at 85kts/1000fpm instead of my normal 70kts/1500fpm. Despite this the engine started to seem a little rough by about 1500 feet so lowered the nose further and powered back a bit. This wasn’t enough to fix the issue and the engine got so rough that I actually thought it was going to quit this time. It was doing about 3800RPM on full power. I pulled the power right back to idle and it smoothed out, and brought it back to the field without powering back up. By the time I was on the ground it had cooled off a bit and did a ground run-up without any issues. Obviously I haven’t fixed the problem so wanted to see if any of you had any ideas. Some relevant info:
The engine has run hot many times in the past while test flying the plane as I was sorting out the cowling, air intakes, radiator size etc. During test flights I was often on the upper limits of CHT and oil temperatures but the engine never missed a beat. The engine is running much cooler now yet this issue is new.
The issue seems to be exclusively on hot days, after high power use such as takeoff climb. Despite it seeming to be directly related to heat the gauges don’t show any significant increase in temperatures compared to other days when I don’t have issues. The cowl is tight and the engine does normally run a bit hot. CHT is often about 110C/ 230F on takeoff so I use waterless coolant. This has never been an issue until now, and on these hot days the temperatures are only up a few degrees from this.
The oil pressure and fuel pressure remain normal at all times. The carbs have a couple hours on them since new and have not been adjusted from factory settings. Plugs today were brand new but the ones I took out did not look fouled in any way.
Previously when the engine has just run a bit rough but not alarmingly, I was able to power back for a few minutes and let the engine cool a bit and then it would power back up without issue. I didn’t attempt this today due to the severity of the rough running.
Any ideas would be helpful and appreciated. I can cut up the cowling and get the CHTs and oil temps down at the expense of some cruise speed if I need to, but I would still like to know what is causing this when it was running fine before with the same set-up. Thanks!
